The present invention relates to a sheet like object conveying method and apparatus for conveying a sheet like object to a predetermined position, and more particularly to an improvement of its conveying sequence. It also relates to a structural example of the sheet like object conveying apparatus of the invention realized in an electrophotographic apparatus or the like as a paper conveying apparatus, and a printer using the same.
The paper conveying sequence in a recent electrophotographic apparatus in which a paper conveying apparatus is incorporated as an important constituent element often fails to convey the paper normally due to such factors as high speed processing and increase of paper variations (various paper qualities, thicknesses). A more specific description is given below.
FIG. 1 is a schematic longitudinal sectional view showing an internal structural example of a general electrophotographic printer having a paper conveying apparatus as a conventional sheet like object conveying apparatus. In FIG. 1, a paper cassette 1 is disposed beneath the printer, and a developing device 3 and a fixing device 4 are disposed thereabove.
A photosensitive drum 31 is provided in the developing device 3, and a pair of heating and fixing rollers 41 is disposed in the fixing device 4. A pair of exit rollers ER is disposed near a paper outlet of the fixing device 4.
A pickup roller PR is disposed in the paper cassette 1. This pickup roller PR takes out uppermost one sheet from a stock of sheets stacked up in the paper cassette 1. Near the paper inlet of the developing device 3, a pair of resist rollers RR is disposed. Plural pairs of feed rollers FR1, FR2, . . . are disposed between the pickup roller PR and pair of resist rollers RR, and by these plural pairs of feed rollers FR1, FR2, . . . , pair of resist rollers RR, photosensitive drum 31, pair of heating and fixing rollers 41, and pair of exit rollers ER, the paper conveying path is formed as indicated by single dot chain line in FIG. 1. In the example shown in FIG. 1, there are three pairs of feed rollers FR1, FR2, FR3, but this is only an example and it is not limited particularly.
In the printer having the paper conveying apparatus as the conventional sheet like object conveying apparatus as shown in FIG. 1, the operation is as follows. First, one sheet is taken out from the paper cassette 1 by the pickup roller PR, and is conveyed into the developing device 3 through the feed rollers FR1, FR2, FR3 and resist rollers RR. On the other hand, data given from a host apparatus not shown (for example, personal computer or word processor) is emitted on the photosensitive drum 31 of the developing device 3 by the optical unit 2, and a latent image is formed. The latent image formed on the photosensitive drum 31 is developed by the toner to be a sensible image, which is transferred on the paper, and it is further conveyed to the fixing device 4 to be heated and fixed, and is discharged into a discharge tray 7 through the pair of exit rollers ER.
The paper conveying sequence from the paper cassette 1 to the developing device 3 in the paper conveying apparatus of the above case is described in detail below.
When paper conveying is instructed, the pickup roller PR, and feed rollers FR1, FR2, FR3 are driven to rotate at predetermined speeds, but the pair of resist rollers RR is not driven and remains stopped. In such state, the paper taken out from the paper cassette 1 by the pickup roller PR is conveyed in the sequence of the feed rollers FR1, FR2, FR3, and is conveyed until colliding with the pair of resist rollers RR in stopped state. The reason why the pair of resist rollers RR is not rotating herein is that collision of the paper with the pair of resist rollers RR in stopped state causes to deflect the paper of the pressing force of the pair of feed rollers FR3 immediately at the upstream side, so that the leading end of the paper is collided with the contacting portion of pair of resist rollers RR to correct the inclination uniformly. It hence must avoid passing-through of the paper through the contacting portion of the pair of resist rollers RR in stopped state, and arrest the paper securely. However, to arrest running of paper by the pair of resist rollers RR, it is a required condition that the paper conveyed at the paper conveying speed (for example, 627 mm/sec) of the feed roller FR immediately at the upstream side of the resist roller RR should be arrested.
After being aligned by the pair of resist rollers RR, the paper is conveyed at a process speed (speed for developing, transferring and fixing, for example, 180 mm/sec) slower than the speed being conveyed up to the pair of resist rollers RR (for example, 627 mm/sec) so as to be developed, transferred and fixed.
In this way, in the paper conveying sequence of the paper conveying apparatus as the conventional conveying apparatus of sheet like object, the inclination of the paper end is aligned uniformly by deflecting by colliding the leading end of the paper in the midst of conveyance with the contacting portion of the pair of resist rollers in stopped state at a relatively high speed, and then it is conveyed at a relatively low speed. Such prior art is disclosed, for example, in Japanese Patent Application Laid-open No. 1-299139 (1989).
Meanwhile, as mentioned above, the paper conveying sequence of the paper conveying apparatus in the recent electrophotographic apparatus is required to be higher in paper conveying speed and enhanced in the conveying capacity, in conditions including high speed processing and increase in paper variation (various paper qualities and thicknesses). However, when the paper conveying speed is fast or the paper is thick, the motion energy of the paper in the conveying process is considerably large, and the paper may not be arrested at the contacting portion of the pair of resist rollers in stopped state, and may sometimes pass through the contacting portion of the pair of resist rollers RR.
As a result, there are many problems such that the paper end may slip out of the pair of resist rollers to cause an error in the print starting position, or desired deflecting amount may not be controlled when deflecting by colliding the paper with the pair of resist rollers, impact noise may occur when the paper collides with the pair of resist rollers, especially in the case of thick paper, or the paper may run obliquely after passing through the pair of resist rollers.